1. Field of the Invention
The present invention relates to a mechanically operable electrical contacting device utilized for producing switches or relays, for example. The present invention also relates to a method of making such an electrical contacting device.
2. Description of the Related Art
Mechanically operable contacting devices, used for e.g. switches and relays, are designed to close and open an electrical circuit by touching two contacts to each other and separating them. Switches or relays incorporating such a contacting device are used in various applications since the current path of a circuit can be completely broken by bringing the contacting device into circuit-open position, in which the paired contacts are spaced apart from each other, with the air (insulator) intervening therebetween. Such reliable switching devices in use are found in information equipment, industrial machines, automobiles and home electric appliances, for example.
FIGS. 19 and 20 shows a conventional electrical contacting device X5 of the mechanically operable type described above. The contacting device X5 consists of a movable unit (first contactor) 71 and a stationary unit (second contactor) 72.
The movable unit 71 includes a conductive blade 73, a contact 74 disposed at one end of the blade 73, and a socket 75 secured to the blade 73. Such an arrangement is sometimes referred to as a “single contact structure”, in which a single contact (74) is provided on one conductive blade (73). While the contact 74 is formed of a conductive material, the socket 75 is formed of an insulating material (resin, for example). The conductive blade 73 is, at the other end, electrically and mechanically connected to a lead 76 made of braided copper wires. The lead 76 is connected to a non-illustrated external circuit. A pin 77 extends through the socket 75 so that the movable unit 71 is allowed to pivot about the axis of the pin 77. The pin 77 is fixed to a non-illustrated case. The pivot of the movable unit 71 is effected by a driving mechanism (not shown) provided with a solenoid, for example.
The stationary unit 72 includes a conductive blade 78 and a contact 79 made of a conductive material. The blade 78 is connected to a non-illustrated external circuit. The contact 79 is located on the track of the contact 73 of the pivoting unit 71.
With the above arrangement, the movable unit 71 is caused to pivot toward the stationary unit 72, with a prescribed voltage applied to the electrical contacting device X5. Then, when the contacts 74 and 79 touch each other, as shown in FIG. 20, electric current flows, for example, from the conductive blade 78 to the lead 76 via the contacts 79, 74 and the blade 73. When the movable unit 71 is caused to pivot in the direction spacing away from the stationary unit 72, the contacts 74 and 79 are separated, as shown in FIG. 19, whereby the electrical current stops.
As is known in the technical field of contacting devices, when the current flowing through the closed contacts is greater than a prescribed threshold (“minimum discharge current”), or when the potential difference between the closed contacts is greater than a prescribed threshold (“minimum discharge voltage”), arc discharge will occur between the contacts as they part from each other.
Specifically, suppose that a current greater than the prescribed threshold is flowing through the closed contacts. As these contacts are parting from each other, the contact area between them gradually decreases, whereby the current flowing through the contacts will concentrate. Accordingly, heat is generated at the contacts, and the surface of the contacts begins to melt. While the separation between the contacts is small, a bridge made of molten contact material is formed between the contacts, thereby keeping the contacts electrically connected to each other. The bridge produces a vapor of metal, and arc discharge occurs through the vapor. Then, the arc discharge causes the ambient air to glow. Further, when the contacts are separated by a sufficient distance, the arc discharge will cease.
FIG. 21 is a graph showing how the occurrence probability of arc discharge depends on the current flowing through paired contacts. For this graph, the contacts made of gold were initially held in pressing contact with each other under prescribed pressing force (10 mN, 100 mN and 200 mN). While a constant voltage of 36V was being applied between the contacts, the contacts were brought away from each other. The occurrence probability of arc discharge was plotted. With a 36V-constant voltage source connected to the contacts, the supplied electric current was adjusted by changing the resistance of a resistor connected in series to the contacts. The substantial contact area for the paired contacts may be no greater than several ten μm2. The abscissa of the graph represents the current passing through the closed contacts, while the ordinate represents the occurrence probability of arc discharge. Under any one of the pressing forces, the occurrence probability of arc discharge becomes substantially 100% when the passing current is no smaller than 0.6 A. On the other hand, the occurrence probability becomes substantially 0% when the passing current is no greater than 0.1 A. More detailed information relating to this graph can be found in following non-patent document 1:
[Non-Patent Document 1]
Yu Yonezawa and Noboru Wakatsuki, “Japanese Journal of Applied Physics”, The Japan Society of Applied Physics, Jul. 2002, Vol. 41, Part 1, No. 7A, p. 4760–4765.
The graph of FIG. 21 shows that the minimum discharge current (minimum arc current) Imin required for causing arc discharge is in a range of 0.1–0.6 A. It is known that the minimum discharge current depends on the kind of material. Likewise, a minimum discharge voltage (minimum arc voltage) Vmin for causing arc discharge can be determined, and it depends on the kind of material. According to a report, the minimum discharge current Imin for contacts made of gold is 0.38 A, and the minimum discharge voltage Vmin is 15V. It should be noted that the actually measured Imin or Vmin is not always constant and may be subject to variation due to the influence from the electrical field in the space between the paired contacts or from the surface condition of the contacts.
When the electrical contacting device X5 is closed, all the current required by a load (non-illustrated, external circuit for which the current is supplied) passes through the contacts 74 and 79. Thus, when the current to be supplied to the load is greater than the minimum discharge current, arc discharge will occur between the contacts 74 and 79 at the time of contact separation. Generally, the current required by the load is often greater than the minimum discharge current of the contacting device X5.
The generation and disconnection of the arc discharge leads to the melting, evaporation and re-solidification of the material of the contacts 74, 79. Consequently, the contact material will be ablated or transformed, and the contact resistance between the contacts 74 and 79 may be varied. Thus, as the arc discharge between the contacts 74 and 79 occurs more frequently, the reliability of the contacting device X5 tends to deteriorate, and the life of the product tends to be shortened. In particular, such reliability deterioration and shortened production life become more serious when the contacting device X5 is used for passing or disconnecting high current.
In the conventional contacting device X5, the contacts 74, 79 include a low-resistance base member made of copper, and a low-resistance and anticorrosive metal coating (e.g. Au, Ag, Pd or Pt) formed over the base member. However, these low-resistance metals have a relatively low melting point. Thus, they tend to melt by the heat resulting from the arc discharge, thereby suffering ablation and transformation. In this regard, use can be made of metals that melt less easily by the heat generated by the arc discharge. However, such metals have relatively high resistance. Thus, it is unpractical to adopt high-melting point metals for producing contacts of the conventional contacting device X5, in which it is essential to achieve a low contact resistance.
For prevention of arc discharge, a spark quencher may be provided on the contacting device X5. A spark quencher may comprise a varistor or diode connected in parallel to the contacts 74, 79. This approach, however, requires for additional elements beside the contacting device X5. Thus, the use of spark quenchers may be unpreferable in light of the device size and production cost.
In the conventional contacting device X5, a proper closed condition may fail to be achieved due to some foreign matter such as dust intervening between the contacts 74 and 79, when the movable unit 71 is caused to pivot for electrical connection. To avoid such an inconvenience, the contacting device X5 may adopt a movable unit 71′ as shown in FIG. 22 in place of the single-contact movable unit 71. The movable unit 71′, including a twin-structure conductive blade 73′, two contacts 74′ provided on one end of the respective branches of the blade 73′, and a socket 75 fitted on the blade 73′, has the so-called “twin-contact structure” whereby a single conductor blade 73′ is provided with two contacts 74′. The conductive blade 73′ is connected electrically and mechanically to a lead 76. Likewise of the movable unit 71, the movable unit 71′ is caused to pivot about a pin 77 secured to a case (not shown).
Electrical contacting devices including such a twin-contact movable unit are disclosed in following patent-documents 1 and 2, for example.
[Patent-Document 1]                Japanese patent laid-open H05-54786        
[Patent-Document 2]                Japanese patent laid-open H10-12117        
In the contacting device X5 with the twin-contact movable unit 71′, foreign matter may intervene between one of the twin contacts 74′ and the lower contact 79, but still the other twin contact can come into conduction with the contact 79 if the foreign matter is not too large. As a result, a desired closed-circuit condition is achieved. However, as in the case where the single-contact movable unit 71 is adopted, arc discharge will occur also in the contacting device X5 provided with the twin-contact movable unit 71′.